Process for manufacturing thin pipes

ABSTRACT

The invention relates to a method for manufacturing thin-walled pipes, which are made of a heat-resistant and wear-resistant aluminum-based material. The method comprises the providing of a billet or a tube blank made of a hypereutectic aluminum-silicon AlSi material, possibly a subsequent averaging annealing, the extruding of the billet or of the tube blank to a thick-walled pipe, and the hot deformation of this pipe to a thin-walled pipe. Such a method is in particular suited for the production of cylinder liners of internal combustion engines, since the produced liners exhibit the required properties in regard to wear resistance, heat resistance and reduction of pollutant emission.

BACKGROUND OF THE INVENTION

The invention relates to a method for manufacturing thin-walled pipes,which pipes are made of a heat-resistant and wear-resistantaluminum-based material, in particular for use as cylinder liners forinternal combustion engines.

Cylinder liners are components subject to wear, which are inserted,pressed or cast into the cylinder openings of the crankcase of theinternal combustion engine.

The cylinder faces of an internal combustion engine are subjected tohigh frictional loads from the pistons or, respectively, from the pistonrings and to locally occurring high temperatures. It is thereforenecessary that these faces be made of wear-resistant and heat-resistantmaterials.

In order to achieve this goal, there are numerous processes amongstothers to provide the face of the cylinder bore with wear-resistantcoatings. Another possibility is to dispose a cylinder liner made of awear-resistant material in the cylinder. Thus, gray-cast-iron cylinderliners were used, amongst others, which liners however exhibit a lowheat conductivity as compared to aluminum-based materials and exhibitother disadvantages.

The problem was first solved with a cast cylinder block made of ahypereutectic aluminum-silicon AlSi alloy. The silicon content islimited to a maximum of 20 weight-percent for reasons associated withcasting technology. As a further disadvantage of the casting method itis to be mentioned that primary silicon particles of relatively largedimensions (about 30-80 μm) are precipitated during the solidificationof the melt. Based on the size and their angular and sharp-edged form,the primary silicon Si particles lead to wear at the piston and pistonrings. One is therefore forced to protect the pistons and the pistonrings with corresponding protective layers/coatings. The contact face ofthe silicon Si particles to the piston/piston ring is flat-smoothedthrough mechanical machining treatment. An electrochemical treatmentthen follows to such a mechanical treatment, whereby the aluminum matrixis slightly reset between the silicon Si grains such that the silicon Sigrains protrude insignificantly as support structure from the cylinderface. The disadvantage of thus manufactured cylinder barrels lies, onthe one hand, in a substantial manufacturing expenditure (costly alloy,expensive mechanical machining treatment, iron-coated pistons, armoredand reinforced piston rings) and, on the other hand, in the defectivedistribution of the primary silicon Si particles. Thus, there are largeareas in the microstructure which are free of silicon Si particles andthus are subject to an increased wear. In order to prevent this wear, arelatively thick oil film is required as separation medium betweenbarrel and friction partner. The clearing depth of the silicon Siparticles is amongst others decisive for the setting of the oil-filmthickness. A relatively thick oil film leads to higher friction lossesin the machine and to a larger increase of the pollutant emission.

In comparison, a cylinder block according to the DE 42 30 228, which iscast of an below-eutectic aluminum-silicon AlSi alloy and is providedwith liners of a hypereutectic aluminum-silicon AlSi alloy material ismore cost advantageous. However, the aforementioned problems are alsonot solved in this case.

In order to employ the advantages of the hypereutectic aluminum-siliconAlSi alloys as a liner material, the microstructure in regard to thesilicon grains is to be changed. As is known, aluminum alloys, whichcannot be realized using casting technology, can be custom-produced bypowder-metallurgic processes or spray compacting.

Thus, in this way hypereutectic aluminum silicon AlSi alloys areproduceable which have a very good wear resistance and receive therequired heat resistance through alloying elements such, as for exampleiron Fe, nickel Ni, or manganese Mn, based on the high silicon content,the fineness of the silicon particles, and the homogeneous distribution.The primary silicon particles present in these alloys have a size ofabout 0.5 to 20 μm. Therefore, the alloys produced in this way aresuited for a liner material.

Even though aluminum alloys are in general easy to be processed, thedeformation of these hypereutectic alloys is more problematic. A methodfor producing liners from a hypereutectic aluminum-silicon alloy isknown from the German printed patent document EP 0 635 318. According tothis reference the liner is produced by extrusion presses at very highpressures and extrusion rates of from 0.5 to 12 m/min. Very highextrusion rates are required in order to produce the liners to a finaldimension with extruders cost-effectively. It has been shown that thehigh extrusion rates lead to a tearing of the profile during extrusionin case of such difficultly extrudable alloys and of the small wallthicknesses of the liners to be achieved.

SUMMARY OF THE INVENTION

The object of the invention is to provide for an improved,cost-advantageous method for manufacturing thin-walled pipes, inparticular for cylinder liners of internal combustion engines, whereinthe finished liners are to exhibit the required property improvements inregard to wear resistance, heat resistance, and reduction of thepollutant emission.

According to the invention, the object is solved by a method with themethod steps recited in patent claim 1.

Additional embodiments of the invention are given in the sub-claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the microstructure of a spray compacted billet.

FIG. 2 shows the microstructure of a pipe formed by annealing and hotextrusion.

FIG. 3 shows the microstructure of a spray compacted billet.

FIG. 4 shows the microstructure of a pipe formed by hot extusion.

DESCRIPTION OF THE INVENTION

The required tribological properties are in particular achieved in thatsilicon particles are present in the material as primary precipitates ina size range of from 0.5 to 20 μm, or as admixed particles in a sizerange of up to 80 μm. Methods have to be employed for the manufacture ofsuch aluminum Al alloys which allow a substantially highersolidification rate of a high-alloy melt than it is possible withconventional casting processes.

On the one hand, the spray compacting method (in the following referredto as "spray compacting") belongs to this. An aluminum alloy melt,highly alloyed with silicon, is atomized and cooled in the nitrogenstream at a cooling rate of 1000° C. The in part still liquid powderparticles are sprayed onto a rotating disk. The disk is continuouslymoved downwardly during the process. A cylindrical billet is generatedby the superposition of the two motions, wherein the billet hasdimensions of from approximately 1000 to 3000 in length at a diameter ofup to 400 mm. Primary silicon Si precipitates up to a size of 20 μm aregenerated in this spray compacting process based on the high coolingrate. An adaptation of the silicon Si precipitate size is achieved withthe "gas to metal ratio" (standard cubic meter of gas per kilogram ofmelt), with which the solidification speed can be set in the process.Silicon contents of the alloys up to 40 weight-percent can be achievedbased on the solidification rates and the supersaturation of the melt.The supersaturation state in the resulting billet is quasi "frozen"based on the fast quenching of the aluminum melt in the gas stream.

Alternatively to the billet manufacture, also thick-walled tube blankshaving inner diameters of from 50-120 mm and a wall thickness up to 250mm can be manufactured with the spray compacting. For this purpose, theparticle stream is directed after the atomization onto a support piperotating horizontally around its longitudinal axis, and is compactedthere. Based on a continuous and controlled advance in horizontaldirection, a tube blank is produced in this way, which tube blank servesas stock blank for the further processing by tube extrusion pressesand/or other hot-deformation processes. The aforementioned support pipeis made of a conventional aluminum wrought alloy or of the same alloy,as it is manufactured by the spray compacting (of the same kind).

The spray compacting process in addition offers the possibility to enterparticles with a particle injector into the billets or into the tubeblanks, which particles were not present in the melt. There exists aplurality of adjustment possibilities for a microstructure since theseparticles can exhibit any desired geometry and any desired size between2 μm and 400 μm. These particles can be, for example, silicon Siparticles in the range of from 2 μm to 400 μm or oxide-ceramic particles(for example, Al₂ O₃) or non-oxide-ceramic particles (for example, SiC,B₄ C, etc.) in the aforementioned particle-size spectrum, as they arecommercially available and sensible for the tribological aspect.

A further possibility to produce a suitable microstructure formationlies in the fast solidification of an aluminum alloy melt,supersaturated with silicon (in the following "powder route"). For thispurpose, a powder is produced by means of an air atomization orinert-gas atomization of the melt. This powder can on the one hand becompletely alloyed, which means that all alloy elements were containedin the melt, or the powder is mixed from several alloy powders orelement powders in a subsequent step. The completely alloyed powder orthe mixed powder is subsequently pressed by cold-isostatic pressing orhot pressing or vacuum hot-pressing to a billet or a thick-walled hollowcylinder (tube blank).

The microstructural condition of the spray-compacted billets/tube blanksor of the billets/tube blanks which were manufactured via the powderroute can be changed with subsequent averaging annealing processes. Themicrostructure can be set with an annealing to a silicon grain size offrom 2 to 30 μm as it is desired for the required tribologicalproperties. The growing of larger silicon Si particles during theannealing process is effected by diffusion in the solid at the expenseof smaller silicon particles. This diffusion is dependent on theoveraging and annealing temperature and the duration of the annealingtreatment. The higher the temperature is chosen, the faster the siliconSi grains grow. Desired temperatures are at about 500° C., wherein anannealing time period of 3 to 5 hours is sufficient.

The thereby resulting and therefore custom-made microstructure no longerchanges in the subsequent processing steps or it changes favorably forthe required tribological properties.

A thick-walled pipe with a wall thickness of from 6 to 20 mm is formedfrom the billet blank, where the billet blank was manufactured by "spraycompacting" or by the "powder route", by hot deformation, preferably byextrusion. For this purpose, the extrusion temperatures are between 300°C. and 550° C.

The extruding not only serves to form, but also to close the residualporosity of the spray-compacted billets or of the spray-compacted tubeblanks (1-5%) or, respectively, of the billets or of the tube blankswhich were manufactured via the "powder route" (1-40%), and tocompletely and finally consolidate the material.

The additional, still necessary reduction in wall thickness is achievedby swaging or another hot-deformation process at temperatures of from250° C. to 500° C.

The pipe, formed to the final wall thickness, is subsequently cut intopipe sections of the required length.

The invention method has the advantage that the material for the linercan be custom-made. The high expenditure in the case of extruding, bothin regard to extrusion pressure, extrusion rate, as well as productquality, is avoided based on the subsequent second hot-deformationprocess step.

EXAMPLE 1

An alloy of the composition Al₁ Si₂₅ Cu₂.5 Mg₁ Ni₁ is compacted to abillet according to the spray compacting process at a melt temperatureof 830° C. with a gas/metal ratio of 4.5 m³ /kg (standard cubic metergas per kilogram of melt). The silicon Si precipitates in the size rangeof from 1 μm to 10 μm (microstructure FIG. 1) are present under therecited conditions in the spray-compacted billet. The spray-compactedbillet is subjected to an annealing treatment of four hours at 520° C.The silicon Si precipitates are in the size range of from 2 μm to 30 μmafter this annealing treatment. A pipe with an outer diameter of 94 mmand an inner diameter of 69.5 mm (microstructure FIG. 2) is produced ina porthole die by hot extruding at 420° C. and a profile exit rate of0.5 m/min. The subsequent hot deformation by round kneading and swagingat 420° C. from an outer diameter of 94 mm to an outer diameter of 79 mmand an inner diameter of 69 mm, which is formed by a mandrel, does notlead to a change in microstructure.

EXAMPLE 2

An alloy of the composition Al₁ Si₈ Fe₃ Ni₂ is compacted at a melttemperature of 850° C. of the hot metal with a gas/metal ratio of 2.0 m³/kg after the spray compacting process to a billet. 20% Si particles inthe size range of from 40 μm to 71 μm are added to this alloy with theparticle injector. A homogeneous microstructure can be produced based onthe process (microstructure FIG. 3). Since the desired microstructureresulted with the spray-compacting process, an annealing treatment isnot required. A pipe having an outer diameter of 94 mm and an innerdiameter of 69.5 mm (microstructure FIG. 4) resulted from the hotextrusion at 450° C. and a profile discharge speed of 0.3 m/min in aporthole die. The subsequent hot deformation by round kneading andswaging at 440° C. from an outer diameter of 94 mm to an outer diameterof 79 mm does not lead to a change in microstructure.

EXAMPLE 3

An alloy of the composition Al₁ Si₂₅ Cu₂.5 Mg₁ Ni₁ is atomized with airat a melt temperature of 830° C. of the hot metal. The resulting powderis collected and cold-pressed isostatically at 2700 bar to a billethaving an outer diameter of 250 mm and a length of 350 mm. The densityof the billet amounts to 80% of the theoretical density of the alloy.The primary silicon Si precipitates are in the range of from 1 μm to 10μm. The isostatically cold-pressed billets are subjected to an annealingtreatment of four hours at 520° C. After this annealing treatment, thesilicon Si precipitates are in the size range of from 2 μm to 30 μm. Thematerial is completely compacted and formed to a pipe having an outerdiameter of 94 mm and an inner diameter of 69.5 mm based on the hotextrusion at 420° C. and a profile discharge speed of 0.5 m/min in aporthole die. The subsequent hot deformation by round kneading andswaging at 420° C. from an outer diameter of 94 mm to an outer diameterof 79 mm and an inner diameter of 69 mm, which is formed by a mandrel,does not lead to a change in microstructure.

EXAMPLE 4

An alloy of the composition Al₁ Si₂₅ Cu₂.5 Mg₁ Mi₁ is compacted at amelt temperature of 850° C. of the hot metal with a gas/metal ratio of2.5 m³ /kg according to the spray-contacting method to a tube blankhaving an outer diameter of 250 mm and an inner diameter of 80 mm. Forthis purpose, a thin-walled pipe, having an outer diameter of 84 mm andhaving a wall thickness of 2 mm and made of a conventional aluminumwrought alloy (AlMgSi₀.5), serves as rotating support pipe onto whichthe above recited alloy is sprayed. The silicon precipitates are in thesize range of from 0.5 μm to 7 μm in the spray-compacted tube blankunder the recited conditions. In order to set the silicon precipitatesto a size of from 2 to 30 μm, the spray-compacted tube blank issubjected to an annealing treatment of 5 hours at 520° C. A pipe havingan outer diameter of 94 mm and an inner diameter of 69.5 mm results bytube extrusion at 400° C. and a profile discharge speed of 1.5 m/min. Inthis case, the pipe support material AlMgSi₀.5 in particular has apositive effect on the required extrusion force and speeds since it actsas lubricant in the direction of and parallel to the mandrel. Thesubsequent hot deformation by round kneading and swaging at 430° C. froman outer diameter of 94 mm to an outer diameter of 79 mm and an innerdiameter of 69 mm, which is formed by a mandrel, does not lead to achange in microstructure.

We claim:
 1. A method for manufacturing liners for internal combustion engines made of a hypereutectic aluminum silicon AlSi alloy comprising the steps of spray compacting an Al alloy melt to obtain starting structures, wherein the contained primary silicon Si particles have a size of from 0.5 to 20 μm; maintaining the starting structures at an extrusion temperature of from about 300 to 550° C.; extruding the starting structures to thick-walled pipes having a wall thickness of from 6 t 20 mm; and reducing the wall thickness of the thick-walled pipes by a hot-deformation process at temperatures of from 250 to 500° C. from 1.5 to 5 mm.
 2. The method according to claim 1, wherein the starting structures are billets.
 3. The method according to claim 1, wherein the starting structures are tube blanks.
 4. The method according to claim 1, wherein the contained primary silicon Si particles have a size of from 1 to 10 μm.
 5. The method according to claim 1, further comprisingannealing said starting structures in case of need for coarsening the contained primary silicon Si particles to overage them for growing the primary silicon Si particles to a size of from about 2 to 30 μm.
 6. The method according to claim 1, wherein the Al alloy melt employed for manufacturing the starting structures has about the following composition:AlSi(17-35)Cu(2.5-3.5)Mg(0.2-2.0)Ni(0.5-2).
 7. The method according to claim 1, wherein thealloy melt employed for manufacturing the starting structures has about the following composition: AlSi(17-35)Fe(3-5)Ni(1-2).
 8. Method according to claim 1, wherein the Al alloy melt employed for manufacturing the starting structures has about the following composition:AlSi(25-35).
 9. The method according to claim 1, wherein the Al alloy melt employed for manufacturing the starting structures has about the following composition:AlSi(17-35)Cu(2.5-3.3)Mg(0.2-2,0)Mn(0.5-5).
 10. The method according to claim 1, further comprising whereinspray compacting the Al alloy melt further comprises;furnishing a part of the silicon Si from a melt of an aluminum-silicon AlSi alloy employed for that purpose into the starting structure; and furnishing a part of the silicon in the form of silicon Si powder by means of a particle injector into the starting structure during spray compacting.
 11. The method according to claim 1, further comprisingannealing said starting structures at temperatures of from about 460 to 540° C. over a time period of from about 0.5 to 10 hours in case of need for coarsening the contained primary silicon Si particles to overage them for growing the primary silicon Si particles to a size of from about 2 to 30 μm.
 12. The method according to claim 1, further comprisingperforming the hot-deformation process of the thick-walled pipes by round kneading and swaging or rotary swaging.
 13. The method according to claim 1, further comprisingperforming the hot-deformation process of the thick-walled pipes by tube rolling with an internal tool.
 14. The method according to claim 1, further comprisingperforming the hot-deformation process of the thick-walled pipes by press rolling.
 15. The method according to claim 1, further comprisingthe hot-deformation process of the thick-walled pipes by tube drawing.
 16. The method according to claim 1, further comprisingperforming the hot-deformation process of the thick-walled pipes by annular rolling.
 17. The method according to claim 1, further comprisingcutting the pipes into pipe sections of a desired length after having been formed in diameter and in wall thickness to a final dimension.
 18. A method for manufacturing liners for internal combustion engines made of a hypereutectic AlSi alloy comprising the steps ofcompacting metallic powder obtained by atomization in a particle size of less than about 250 μm, wherein contained primary silicon Si particles have a size of from about 0.5 to 20 μm to obtain starting structures; maintaining the starting structures at an extrusion temperature of from about 300 to 550° C.; extruding the starting structures to thick-walled pipes having a wall thickness of from 6 to 20 mm; and reducing the wall thickness of the thick-walled pipes by a hot-deformation process at temperatures of from 250 to 500° C. from 1.5 to 5 mm.
 19. The method according to claim 18, further comprisingcompacting the metallic powder by hot compacting.
 20. The method according to claim 18, further comprisingcompacting the metallic powder by cold compacting.
 21. The method according to claim 18, wherein the metallic powder is a member selected from the group consisting of metal powder, alloy powder, and mixtures thereof obtained by atomization in a presence of a member selected from the group consisting of inert gas, air, and mixtures thereof.
 22. Method for manufacturing liners for internal combustion engines made of a hypereutectic aluminum silicon AlSi alloy, characterized in thatbillets or tube blanks are provided by spray compacting an alloy melt or by hot compacting and cold compacting, respectively, a mixture of metal powder or alloy powder, obtained by air atomization or inert-gas atomization, respectively, in a particle size of smaller than 250 μm, wherein the contained primary silicon Si particles have a size of from 0.5 to 20 μm, and preferably a size of from 1 to 10 μm, said billets or tube blanks are subjected to an overaging annealing, wherein the primary silicon Si particles grow to a size of 2 to 30 μm, the billets or tube blanks, kept at an extrusion temperature of from 300 to 550° C., are extruded to thick-walled pipes having a wall thickness of from 6 to 20 mm, and the wall thickness of the thick-walled pipes is reduced by a hot-deformation process at temperatures of from 250 to 500° C. from 1.5 to 5 mm. 